In an electric power steering apparatus that drives a motor for steering assistance in accordance with a rotational operation of a steering member such as a steering wheel and transmits the rotational force of the motor to a steering mechanism thus to assist the steering, a steering torque applied to the steering member has to be detected for the drive control of the motor for steering assistance, and for such detection a torque detecting apparatus has been conventionally employed halfway of a steering shaft connecting the steering member and the steering mechanism.
In the torque detecting apparatus, the steering shaft, which is the object of the detection, is split into a first shaft and a second shaft coaxially connected via a torsion bar of a reduced diameter, so that when a steering torque is applied to the steering shaft by a rotational operation of the steering member, a relative angular displacement is created between the first and the second shaft along with distortion of the torsion bar caused by the effect of the steering torque, and the steering torque is detected based on the relative angular displacement.
The detection of the relative angular displacement between the first and the second shaft has been executed by various means, one of which is found in a torque detecting apparatus that includes a cylindrical magnet that integrally rotates with the first shaft and a magnetic yoke that integrally rotates with the second shaft, so as to detect the relative angular displacement utilizing a variation of a magnetic circuit between the cylindrical magnet and the magnetic yoke (For example, refer to Patent Document 1).
The cylindrical magnet that integrally rotates with the first shaft is constituted as a multi-pole magnet including a plurality of magnetic poles circumferentially aligned thereon, and is coaxially fixed to the first shaft. The magnetic yoke that integrally rotates with the second shaft includes a plurality of magnetic pole nails circumferentially aligned on a ring-shaped yoke element so as to axially extend in an axial direction, such that two of such yoke elements with the magnetic pole nails alternately aligned in a circumferential direction constitute a pair, and are positioned such that each of the magnetic pole nails falls on a boundary between the poles (N-pole and S-pole) of the cylindrical magnet.
Under such structure, when the relative angular displacement is created between the first shaft and the second shaft, the circumferential positional relationship of the magnetic pole nails of the paired magnetic yokes that integrally rotate with the second shaft changes with respect to the magnetic poles of the cylindrical magnet that integrally rotates with the first shaft, in an opposite direction to each other, which creates a change in magnetic flux generated in the respective magnetic yokes, and detecting such change by a magnetic sensor constituted of a magnetosensitive element such as a hall element enables detecting the torque applied to the first and the second shaft (steering torque).
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-149062